Active heave compensation system and method

ABSTRACT

An active heave compensation system comprises a motor generator to interact with a load so as to drive the load in a first part of a heave motion cycle and to regenerate at least part of energy with which the load has been driven in a second part of the heave motion cycle, and an electrical storage element for storing the regenerated energy. The active heave compensation system further comprises a power supply electrically connected to the motor generator and the electrical storage element for providing electrical power to at least the motor generator, and a control unit configured to control the power supply substantially in synchronism with the heave motion.

The invention relates to an active heave compensation system, a vesselcomprising such active heave compensation system and an active heavecompensation method.

Heave compensation has been known for many years. Many solutions havebeen provided, some of which will be discussed below. In general, heavecompensation provides for a compensation of heave motion on a load, theheave motion as a result of wave motion. The load may be submerged orpartially submerged, thereby being subjected to the wave motion. Also,or instead thereof, it may be the case that the load is held by afloating platform (such as a vessel), which is subjected to the wavemotion. Further, many other cases may be imaginable where heave motionmay be desired, such as a situation where a load is to be taken from orplaced on a floating platform, the floating platform being subjected towave motion. Heave compensation may be provided for any kind of load,e.g. a load to be carried by a crane or other lifting installation,constructions submerged under water such as pipeline laying equipment,etc. It is to be understood that the above examples are for illustrationonly, and are not intended to limit the scope of this document in anyway.

Heave compensation systems can be subdivided in active and passive heavecompensation systems. Combinations of active and passive systems may beprovided too. In a passive heave compensation system, a compressiblemedium is provided in a form of a gas spring, hydraulic system, etc. toprovide for a compensation. In an active heave compensation system anactuator is provided to actively compensate for effects of the heavemotion. Many constructions have been described in the literature. Ingeneral, in an active heave compensation system, use is made of ahydraulic system. As an example, a hydraulic cylinder may be providedwhich extends and compresses synchronously with the heave motion,thereby interacting with for example a cable holding the load. In eachwave, energy is to be supplied to the hydraulic system to exert a forceonto the load. Some of the energy may be regained in the other part ofthe heave motion cycle and e.g. stored by compression of a gas. In thenext cycle, the compressed gas can then be applied to drive the load orat least to contribute thereto.

Although hydraulic/gas pressure active heave compensation has beenextensively used in many configurations, a disadvantage is that thissetup leads to a complex system and involves a risk of leakage ofhydraulic fluid, resulting on the one hand in a relatively complex andcostly system, while on the other hand requiring regular and securemaintenance to avoid leakage and risks of environmental pollution causedthereby.

Electric heave compensation has been disclosed in WO2009/120066A1. Anelectric motor drives a load in a first part of a heave motion cycle.Energy is then regenerated by a generator (the motor that acts as agenerator) and stored in an electrical storage, such as a capacitor orsuper capacitor. It is however noted that the solutions as presentedtherein to control the charging and discharging of a capacitor or supercapacitor that acts as an electrical storage element require abidirectional converter between the motor-generator or power line on theone hand and the capacitor or super capacitor on the other hand.

Accordingly, it is an object of the invention to provide an active heavecompensation whereby the bidirectional converter between the energystorage or power line on the one hand and the motor-generator on theother hand, may be omitted

According to an aspect of the invention, there is provided an activeheave compensation system comprising:

-   -   a motor generator to interact with a load so as to drive the        load in a first part of a heave motion cycle and to regenerate        at least part of energy with which the load has been driven in a        second part of the heave motion cycle,    -   an electrical storage element for storing the regenerated        energy,    -   a power supply electrically connected to the motor generator and        the electrical storage element for providing electrical power to        at least the motor generator, and    -   a control unit configured to control the power supply to let an        output voltage of the power supply substantially follow, in        synchronism with the heave motion cycle, a charging and        discharging voltage cycle of the electrical storage element.

The active heave compensation system accordingly comprises a combinationof a motor generator and an electrical storage element. In a first partof the heave motion cycle, the motor-generator acts as a motor anddrives the load. In a second part of the heave motion cycle, energy isregained and the motor-generator acts as a generator therebyregenerating at least part of the energy with which the load has beendriven in the first part of the heave motion cycle. The regeneratedenergy is stored in the electrical storage element. The stored energycan now be used in a first part of a following heave motion cycle topower the motor-generator. Within the scope of the invention, for themotor generator, use can be made of a separate motor and a separategenerator which both interact with the load, however in an advantageousembodiment, use is made of a motor type which acts as a generator, thusa motor which, when not provided with electrical energy, but whenmechanically driven by a corresponding motion of the load, generateselectrical energy thereby acting as a generator.

Any type of motor generator may be provided, as an example, use may bemade of an three-phase asynchronous motor. The term motor generator mayin general terms be defined as an arrangement which is adapted toconvert electrical energy into motion and to convert motion intoelectrical energy. The motor generator may comprise a suitable driversuch as a so called motor inverter unit that converts a direct current(DC) supply voltage into a switched, alternating electric supply to themotor-generator and vice versa. The power supply may comprise anyelectrical power delivering device, such as a generator. The powersupply may obtain electrical energy from a power grid, such as a powergrid of a vessel, e.g. an alternating current (AC) power grid. In caseof an alternating power grid, the power supply may for example comprisea so called inverter supply unit to convert electrical energy from thealternating current power grid into a direct current (DC) electricalsupply. An output of the power supply may be connected to a power line,such as a direct current (DC) power line, the power supply to supplyelectrical power to the power line. The motor generator (directly or viaits drive unit, such as the motor inverter unit MIU) may be electricallyconnected to the power line. The electrical storage element may each beelectrically connected to the power line. As a result, the electricalstorage element (also referred to as energy storage element, electricalstorage, energy storage or similar wording) may be connected to thepower line without the intervention of a converter to convert a powerline voltage into an operational voltage of the electrical storageelement, as will be explained below. In accordance with the invention,in order to make the electrical storage element store and releaseelectrical energy in a heave motion cycle, the power supply iscontrolled substantially in synchronism with the heave motion. Thereby,the power supply can be controlled so as to make the electrical storageelement to store energy in a part of the heave motion cycle and torelease electrical energy in another part of the heave motion cycle. Asan example, an output voltage of the power supply can be controlled: ifthe voltage is raised, the energy storage element (e.g. the supercapacitor) is charged thereby storing energy, while if the voltage islowered, such energy storage element is discharged. The output voltageof the power supply may hence (e.g. under control of the control unit)be raised in a part of the heave motion cycle wherein the motorgenerator regenerates energy, so that this energy, or at least a partthereof, is stored in the electrical storage element. In another part ofthe wave motion cycle, when the motor generator drives the load, henceconsumes electrical energy, the power supply may be controlled to lowerthe power supply voltage, so that the electrical storage element isdischarged and the energy released by discharging is at least partlysupplied to the motor generator for driving the load. Hence, insynchronism with the heave motion and/or the operation of the motorgenerator, the power supply causes the electrical storage element tofollow a cycle of charging and discharging, so as to cyclically storeand release electrical energy, and allowing the electrical storageelement to be electrically connected directly to the power line, i.e.without the interposition of a converter, such as a directcurrent-direct current converter.

Any type of electrical storage element can be used, however it ispreferred that a capacitor is applied as a capacitor can provide for alow loss storage, thereby enhancing energy efficiency of the heavecompensation system. Preferably, the capacitor comprises a supercapacitor, as thereby a high capacitance value, and consequently a highenergy storage capacity can be provided in a comparably small volume.Furthermore, a super capacitor may provide for a low series resistance,hence allowing a low loss energy storage, may allow a quick charging anddischarging, may provide a high efficiency, and may provide a longoperating life. Also a combination of a battery and capacitor, such as asuper capacitor can be used as electrical storage element. While acapacitor can provide a high output energy and a battery can provideenergy during a relatively long period, a combination could benefit fromboth features.

In an embodiment, the power supply when controlled by the control unit,may be arranged to provide a substantially constant output current, i.e.an output current that is substantially constant over a heave motioncycle time period. Thereby, a constant load may be provided onto a mainsnetwork, generator or other source of energy from which energy isderived, while the energy storage element (without the intervention of aDC/DC voltage converter) takes care of the buffering of energy in eachcycle of the driving of the load and regeneration of energy

The control unit may further control the motor generator so as to drivethe load in the first part of the heave motion cycle and to regenerateenergy in the second part of the heave motion cycle. The control unit(comprising e.g. a microcontroller, microprocessor, or any programmablelogic device, e.g. being provided with suitable program instructions toperform the actions as described) may thereto e.g. control the motorinverter unit associated with the motor-generator. The control unit maythereby control the motor inverter unit such as to power themotor-generator to drive the load in the first part of the cycle and toregenerate at least part of the energy in the second part of the cycle.

The heave motion may be derived from an output parameter of the powersupply, such as the output current or output power of the power supply,the cyclic driving the load by the motor generator and the regenerationof energy translating into a power demand from the power supply thatfollows such cycles. Thereto, in an embodiment, the control unitcomprises an input connected to the power supply for measuring an outputparameter of the power supply, and an output connected to the powersupply for driving the power supply, the control unit being arranged to

-   -   compare the output parameter of the power supply with a control        unit setpoint, and    -   drive the power supply based on the comparison. Hence, from the        demand of power onto the power supply, a driving (i.e.        controlling) of the power supply is derived so as to allow at        least part of the regenerated energy in the second part of the        wave motion to be stored in the electrical storage element.

As a suitable output parameter, the output parameter of the power supplymay be an output current of the power supply, wherein the output of thecontrol unit is connected to a power supply voltage setpoint input ofthe power supply. Hence, when energy is regenerated, an output currentof the power supply is lowered, and the control unit drives the powersupply so as to increase its output voltage so that the regeneratedenergy from the motor generator can be stored into the electricalstorage element, as the increased power supply output voltage, and hencethe increased electrical storage element voltage results in a flow ofthe regenerated energy (in the form of a charging current) into theelectrical storage element. As a result, the output voltage of the powersupply is cyclically changed substantially in synchronism with the heavemotion, causing the electrical storage element to be cyclically chargedand discharged substantially in synchronism with the power supply outputvoltage cycle, so that the cycle of consumption of energy by the motorgenerator when driving the load and the regeneration of energy issubstantially matched by a corresponding discharging and charging of theelectrical storage element, so as to buffer at least part of theregenerated energy in the electrical storage element without requiring aconverter to interposed between the electrical storage element and themotor generator (or more specifically the motor inverter unit of themotor generator). Due to the controlling of the power supply, a largepower supply output voltage swing may be allowed during the heave motioncycle, so that a large voltage swing may be provided onto the electricalstorage element, allowing a large amount of energy to be stored into andreleased from the electrical storage element.

Alternatively or in addition to measuring a power supply output current,the output parameter of the power supply may be a generator frequencysignal: a high load on the generator translating into a lower generatorfrequency, hence allowing to make use of the generator frequency inorder to obtain a signal substantially representative of the powersupply output current, power supply output power or similar. It is notedthat the measurement of a generator frequency as an alternative tomeasurement of a power supply output current may be applied in anyactive heave compensation system such as the active heave compensationsystem as described in WO2009/120066A1, e.g. an active heavecompensation system comprising a motor-generator to interact with a loadand a control unit which is arranged to control operation of themotor-generator, the control unit being arranged to:

-   -   operate the motor-generator to drive the load in a first part of        a wave motion cycle, and    -   operate the motor-generator to regenerate in a second part of        the wave motion cycle at least a part of the energy with which        the load has been driven in the first part of the wave motion        cycle,    -   the active heave compensation system comprising an electrical        storage element to buffer at least part of the regenerated        energy for powering the motor-generator in a following cycle of        the wave motion.

A suitable value of the control unit setpoint may be determined by thecontrol unit as follows: The control unit may be arranged to determinethe control unit set point from: time averaging an actual power supplyoutput voltage of the power supply, comparing the time averaged actualpower supply output voltage with a power supply output voltage setpoint,and deriving the control unit set point from a result of the comparison.Thereby, the power supply may be controlled so as to deliver an amountof energy that corresponds to an average loss the combination of motorgenerator and electrical storage element, as will be easily understoodfrom the below: The time averaged power supply output voltage, i.e thetime averaged power line voltage, corresponds to the time averagedelectrical storage element voltage, the electrical storage element beingconnected to the power line. In case no energy would be added by thepower supply, the average power supply voltage would lower, as theelectrical storage element would, due to energy losses, get more andmore discharged after each heave motion cycle, so that its (e.g. timeaveraged) operating voltage would lower after each cycle. On the otherhand, if the power supply would supply more energy than would berequired to compensate for losses the time averaged operating voltage ofthe energy storage element would increase causing the power supplyoutput voltage to increase, as the power supply output is, via the powerline, connected to the energy storage element. Hence, as the controlunit keeps the time averaged power supply output voltage at a certainlevel, it is provided that energy losses are compensated.

In addition to or instead of the above embodiment, it is possible thatthe control unit comprises an input connected to the power supply formeasuring a power supply output voltage, and an output connected to thepower supply for driving an output current of the power supply, thecontrol unit being arranged to

-   -   determine a time average of the power supply output voltage,    -   compare the time averaged power supply output voltage with a        control unit setpoint, and    -   drive the output current of the power supply based on the        comparison,

Hence, information about the heave motion is obtained by the controlunit by monitoring the power supply output voltage (which may exhibit aripple substantially synchronous with the heave motion) and controllingthe power supply output current accordingly. An example of suchconfiguration is easily imaginable: in case the power supply has a highoutput impedance, i.e exhibits a current source behavior, the discharingand charging of the electrical storage element translates into a cyclingof the power supply output voltage, due to the charging and dischargingof the electrical storage element. The output current of the powersupply may now be controlled accordingly so as to compensate for energylosses: in case the time averaged power supply output voltage wouldlower, an average energy level of the electrical storage element wouldlower, requiring the power supply to increase its output power to addenergy in order to compensate for the losses, and vice versa. Hence, anextremely simple architecture would be provided allowing to control thecharging and discharging of the electrical storage element at minimumlosses.

The control unit may comprise a heave sensor to measure a heave motion,the control unit being arranged to drive the power supply from themeasured heave motion. Hence, instead of or in addition to deriving theheave motion from the power supply output, a sensor may be applied thatprovides information representative of the heave motion. The sensor maycomprise any suitable sensor, such as an accelerometer that measures anacceleration in the heave motion cycle, a position sensor that measuresa position or position change as a result of the heave motion, etc.Further, any other type of sensor may be applied, such as a camera or an(e.g. infrared, radio-wave or ultrasound) distance measurement unit,etc. that detects a wave motion, the control unit to determine a heavemotion estimate from the detected wave motion. Still further, astationary reference may be applied by the sensor for measurement of theheave motion, e.g. an object (a wind turbine, a drilling platformmounted to a seabed, a satellite system reference, etc.)

The electrical storage element may comprise a super capacitor. The supercapacitor (also referred to as ultra capacitor) may provide for a largecapacitance value hence (given a certain operating voltage range) storea large amount of energy.

The electrical storage element may comprise a plurality of supercapacitors and a voltage balancing to balance an operating voltage ofthe super capacitors, so that imbalances in operating voltage of seriesconnected super capacitors, due to for example spread in capacitancevalue, aging, spread in leakage, etc. may be reduced. The voltagebalancing may for example comprise resistive elements in parallel toindividual or groups of parallel super capacitors, the resistiveelements for example comprising resistors, transistors, such as fieldeffect transistors of which the channel is applied as a resistor, thelatter allowing to control a resistance value by a suitable electricaldrive of its gate.

The super capacitors may be arranged in at least one super capacitorbank, the or each super capacitor bank comprising two parallel circuitboards, the super capacitors of the respective super capacitor bankbeing arranged and extending between the circuit boards. An air flow(forced or passive) may be provided so as to flow along the circuitboards, allowing temperature conditioning of the super-capacitors. Inparticular, an air flow along electrical contacts of the supercapacitors may be provided as the electrical contacts provide for a lowthermal resistance, allowing cooling of the super capacitors.

In order to allow a precise driving of the motor generator so as toapply heave motion compensating forces onto the load in synchronism withthe heave motion cycle and in a correct amount, the control unit maycomprise a sensor for measuring a variable representative of a heavemotion to be compensated, the control unit being arranged to drive themotor generator on the basis of the measured variable. The motorgenerator may be driven directly. Also, it is possible that the motorinverter unit of the motor generator is driven. The sensor may compriseany suitable sensor, such as an accelerometer that measures anacceleration in the heave motion cycle, a position sensor that measuresa position or position change as a result of the heave motion, etc.Further, any other type of sensor may be applied, such as a camera or an(e.g. infrared, radio-wave or ultrasound) distance measurement unit,etc. that detects a wave motion, the control unit to determine a heavemotion estimate from the detected wave motion. Still further, astationary reference may be applied by the sensor for measurement of theheave motion, e.g. an object (a wind turbine, a drilling platform)mounted to a seabed, a satellite system reference, etc.). It is to beunderstood that these measurement principles may not only be applied inthe active heave compensation system as disclosed in this document, butin any active heave compensation system, such as a heave compensationsystem comprising the motor generator to interact with the load and acapacitor, super capacitor or other electrical storage to store at leastpart of the electrical energy regenerated by the motor generator.

Many configurations are possible. The load may comprise an (e.g. solid)vessel motion damping ballast which is movable in a transverse orlongitudinal direction of a vessel so as to at least partiallycompensate a heave motion of the vessel. The vessel may thus compriseone or more motion damping ballasts which are movable in transverserespectively longitudinal direction of the vessel. Suitably moving sucha motion damping ballast in the longitudinal direction may reduce aheave motion of the vessel in the longitudinal direction, and suitablymoving such a motion damping ballast in the transverse direction mayreduce a heave motion of the vessel in the transverse direction. Theactive heave compensation system as described in this document mayaccordingly be applied to drive the motion damping ballast by the motorgenerator in a first part of the heave motion cycle and regenerate atleast part of the energy with which the motion damping ballast has beendriven, in a second part of the heave motion cycle. The regeneratedenergy may then at least partly be stored in the electrical storageelement. Another example may be a crane comprising the active heavecompensation system in accordance with the invention, or a drillingvessel comprising the active heave compensation system in accordancewith the invention. As another example, there is provided a vesselcomprising an active heave compensation system in accordance with theinvention.

According to a further aspect of the invention, there is provided anactive heave compensation method comprising:

-   -   driving the load by a motor generator in a first part of a heave        motion cycle    -   regenerating at least part of energy with which the load has        been driven in a second part of the heave motion cycle,    -   storing the regenerated energy in an electrical storage element,    -   providing by a power supply electrical power to at least the        motor generator, the power supply being electrically connected        to the motor generator and the electrical storage element, and    -   controlling by a control unit the power supply to let an output        voltage of the power supply substantially follow, in synchronism        with the heave motion cycle, a charging and discharging voltage        cycle of the electrical storage element.

With the active heave compensation method according to the invention,the same or similar advantages may be achieved as with the heavecompensation system according to the invention. Also, the same orsimilar embodiments as disclosed in respect of the active heavecompensation system according to the invention, also apply to the activeheave compensation method according to the invention, thereby achievingsame or similar effects as the corresponding embodiment of the activeheave compensation system according to the invention.

Further features effects and advantages of the invention will becomeclear from the appended drawings and corresponding description, in whichnon-limiting embodiments of the invention are disclosed, wherein:

FIG. 1 shows a highly schematic configuration of a load submerged from afloating platform;

FIG. 2 shows a highly schematic heave installation having acompensation;

FIG. 3 shows a highly schematic representation of a wave motion;

FIG. 4 shows a highly schematic representation of a heave motioncompensation system according to the invention;

FIG. 5A shows another embodiment of the heave compensation systemaccording to the invention, while

FIG. 5B shows a block schematic view of a control unit of the heavecompensation system in accordance with FIG. 5A;

FIG. 6A shows another embodiment of the heave compensation systemaccording to the invention, while

FIG. 6B shows a block schematic view of a control unit of the heavecompensation system in accordance with FIG. 7A; and

FIG. 7 shows a schematic cross section of a vessel with solid rolldamping ballast.

It is noted that throughout the figures, the same reference numerals andreference symbols refer to the same or similar items having same orsimilar functions.

FIG. 1 shows a highly schematic view of a partly submerged load L heldby a lifting installation LI such as a crane, the lifting installationLI being positioned on a floating platform FP such as a vessel. The wavemotion will result in vertical forces, thereby providing a periodicvertical movement of the load L as well as the floating platform FP. Asa result thereof, forces will act periodically on the cable CA of thelifting installation LI. The heave compensation is intended tocompensate for the heave motion cycle movements, to thereby avoidpossible damage of the load, overloading the cable CA of the liftinginstallation LI, etc. Although in FIG. 1 an example is shown where boththe load and the platform holding the lifting installation LI are partlysubmerged, it is also possible that one of the load and the liftinginstallation is on shore or mounted to a solid reference, as an examplethe lifting installation may be mounted on a wharf, or the load is to beplaced on the wharf while the lifting installation is mounted on afloating platform. Many other configurations are possible. For example,the load is submerged and is required to be stabilised, while thefloating platform holding the lifting installation is subjected to theheave motion. The cable CA is wound on winch WI. Actuating the winch WIto wind up the cable CA will lift the load L and vice versa.

FIG. 2 highly schematically shows an example of a construction that maybe applied in a conventional heave compensation system again showing thelifting installation LI having a cable CA holding a load L. The cable CAis guided via a pulley wheel PW which is connected to a hydrauliccylinder HC. By downwardly moving a piston PI of the hydraulic cylinderHC, the pulley wheel which is connected to the piston, is also moveddownwardly. Thereby, a length of a loop of the cable CA guided via thepulley wheel PW is altered in length, which will cause the load to belifted respectively lowered depending on the direction of movement ofthe piston Pl. The hydraulic cylinder HC may be actively driven, therebyobtaining an active heave compensation system. Also, or in additionthereto, it is possible that use is made of a gas spring, e.g. formed byan enclosed volume with compressible gas, which acts on a hydraulicsystem of which the hydraulic cylinder HC forms part.

As schematically illustrated in FIG. 3, a wave motion cycle will resultin a periodic pattern of upward and downward forces on either the load,the lifting installation, or both.

FIG. 4 depicts a heave compensation system, comprising a motor generatorMG that interacts with a load (not depicted). The motor generator MG ispowered from a power line PL, such as a direct current (DC) power line.A power supply PS is connected to the power line for providingelectrical power to the power line. The power supply may comprise anelectrical power supply that is arranged to convert for example analternating current mains electrical supply (such an AC power net on avessel) into a direct current supply. Furthermore, the power supply maycomprise a generator. An electrical storage element C, such as acapacitor or super capacitor, is electrically connected to the powerline. The motor generator corn comprise a suitable drive unit, such as aso called motor inverter unit that comprises a plurality of switches inorder to provide a correct polarity from the electrical power supplyline to the motor generator, the motor inverter unit thereby effectivelytransferring the DC supply voltage at the power line into an AC voltagedrive of the motor generator and vice versa during regeneration ofenergy.

In a first embodiment, the power supply is a constant current powersupply, such as a current source. The term constant current is to beunderstood as the power supply providing an output current that issubstantially constant over e.g. a time period of the heave motioncycle. When the motor generator drives the load, a current consumptionof the motor generator exceeds the current supplied by the power supply.Hence, the remainder of the current required for operating the motorgenerator is provided by the electrical storage element, e.g. the supercapacitor which is discharged thereby. During a following part of theheave motion cycle, electrical energy is regenerated, the motorgenerator (e.g. via its motor inverter unit MIU) delivers an electricalcurrent that is stored—together with the current provided by the powersupply, into the electrical storage element. Hence, each heave motioncycle, electrical energy is cycled between the motor generator and theelectrical storage element, i.e. cycled between mechanical energy andelectrical energy each heave motion cycle. Apart from the motor inverterunit as applied in the present embodiment, no electrical conversion isrequired. The electrical power supplied by the power supply is set to aslevel so as to compensate for losses due to dissipation.

In order to accomplish that the power supply delivers a current at amagnitude substantially the same as the losses in the system, so thatthe energy level (and hence the charging) of the electrical storageelement is maintained at a predetermined level, a control of the powersupply may be provided. Thereto, the heave compensation system maycomprise a control unit, an example of which having been depictedschematically in FIG. 5A. FIG. 5A depicts a heave compensation systemwhich is substantially the same as the heave compensation system asdepicted in and described with reference to FIG. 4, however a controlunit CON has been added, which is further detailed in FIG. 5B asdiscussed below. The control unit comprises a control unit input that isconnected to the power line PL (i.e. to the power supply output and theenergy storage element). The control unit CON further comprises acontrol unit output that is connected to the power supply. The controlunit is arranged to measure, at the control unit input an output voltageof the power supply, hence an electrical storage element voltage. Thecontrol unit is arranged to determine a time average of this voltage,for example by means of low pass filter LPF (etc averaging over one ormore heave motion cycle times). The averaged power supply outputvoltage, hence the electrical storage element voltage providesinformation about an energy level as stored in the electrical storageelement: in general: the higher the voltage, the higher the energylevel. Thus, in case the averaged voltage increases, an energy levelincreases, and vice versa. The control unit further comprises areference REF a value of which expresses a desired average voltage leveland a controller CNT such as a proportional controller or a proportionalintegrating controller. The averaged power supply voltage from thefilter LPF and the reference REF are provided to respective inputs ofthe controller. An output of the controller CNT is provided to the powersupply for example to a setpoint input of the power supply that —in thisembodiment—sets the power supply output current. The controller hencebeing arranged to compare the averaged power supply output voltage withthe reference, hence a desired average energy level. In the form of afeedback control mechanism, the power supply is now controlled by thecontrol unit so as to provide an output current that keeps the averagedpower line voltage, hence the (average) energy level in the energystorage element, at a predetermined level. The control unit may comprisefor example a proportional or a proportional/integrating controller. Theconstant output current of the power supply is hence controlled so as tomaintain a level that substantially compensates energy losses.

The invention may not only be implemented using a current source (i.e. ahigh impedance power supply). Embodiments wherein use may be made of alow impedance power supply, will be discussed below.

FIG. 6A schematically depicts a heave compensation similarly to FIGS. 4and 5 comprising a power supply PS, an energy storage element C, a motorgenerator MG in this embodiment comprising a motor inverter unit MIU,and a power line PL. The power line PL electrically interconnecting theenergy storage element C, the power supply PS and the motor generator(in this embodiment its motor inverter unit MIU). The energy storageelement C, the motor generator MG and the power line PL may havefunctions substantially similar or identical as described above withreference to FIGS. 4 and 5 above. The heave compensation system furthercomprises a control unit CON that is further detailed in FIG. 6B.

FIGS. 6A and 6B further depicts a double or combined input control looparchitecture. Firstly, a power supply output current is measured andprovided to the first controller CNT1 of the control unit CON. The firstcontroller CNT compares the measured power supply output current with areference (as will be discussed below). An output of the firstcontroller CNT1 is connected to the power supply namely in thisembodiment a power supply output voltage setpoint input to set an outputvoltage of the power supply. The first control loop hence measures thepower supply output current, compares the power supply output currentwith a reference and drives the output voltage of the power supplyaccordingly. Hence, during each heave motion cycle, when the motorgenerator regenerates energy, the measured power supply current wouldtend to decrease (if the power supply voltage would be kept the same),which makes the first controller CNT1 to increase the power supplyvoltage and vice versa, so as to keep the power supply currentsubstantially constant. Similarly to the embodiment described withreference to FIGS. 5A and 5B, a value of the power supply current is setby the other control loop by setting the reference (setpoint) value atthe input of the first controller CNT1 to a suitable value. This isachieved as follows: similarly as described above with reference toFIGS. 5A and 5B, the power line voltage (which equals the power supplyoutput voltage and the electrical storage element voltage) is measured.The control unit is arranged to measure, at a corresponding secondcontrol unit input an output voltage of the power supply, hence anelectrical storage element voltage. The control unit is arranged todetermine a time average of this voltage, for example by means of lowpass filter LPF (etc averaging over one or more heave motion cycletimes). The averaged power supply output voltage, hence the electricalstorage element voltage provides information about an energy level asstored in the electrical storage element: as explained above, ingeneral: the higher the voltage, the higher the energy level. Thus, incase the averaged voltage increases, an energy level increases, and viceversa. The control unit further comprises a reference REF a value ofwhich expresses a desired average voltage level and a second controllerCNT2 such as a proportional controller or a proportional integratingcontroller. The averaged power supply voltage from the filter LPF andthe reference REF are provided to respective inputs of the secondcontroller CNT2. An output of the second controller CNT2 is provided asa reference to the first controller CNT1. The second controller CNT2hence being arranged to compare the averaged power supply output voltagewith the reference REF, hence expressing a desired average energy level.The first controller now drives the power supply voltage (during theheave motion cycle) so that a substantially constant current is providedby the power supply which current compensates energy losses in the heavecompensation system. Thus, in the form of a feedback control mechanism,the power supply is now controlled by the control unit so as to providean output current that keeps the averaged power line voltage, hence the(average) energy level in the energy storage element, at a predeterminedlevel. The first and second controller may comprise for example aproportional or a proportional/integrating controller. The abovedescribed control loop comprising controller CNT1 which results in asubstantially constant output current of the power supply is hencecontrolled so as to maintain a level that substantially compensatesenergy losses.

An example of a heave motion compensated vessel is described in theinternational patent application PCT/NL2008/000221. It discloses a monohull vessel with a heavy lift crane. In FIG. 7 a schematic cross sectionof the vessel is depicted. The vessel 10 is provided with an active rolldamping mechanism. The active roll damping mechanism comprises a solidroll damping ballast 11 which is movable in the transverse direction ofthe hull (direction indicated by arrow A), a sensor detecting therolling motion of the hull, and a drive and control system 12 operableto cause and control the movements of the solid roll damping ballast inresponse to the detections of the sensor to provide roll stabilization.

The drive and control system may be provided with a heave compensationsystem comprising a motor generator M/G and a energy storage C (such asa super capacitor with a converter) as described above to drive thesolid roll damping mechanism. The movements of the solid roll dampingballast can be described as a cycle, as the ballast may be moved fromlarboard to starboard and vice versa. In the cycle, energy may beproduced and stored in a first part the cycle and may be required inanother part. It is to be understood that the damping ballast may notonly be provided in transverse direction of the vessel, but also orinstead in longitudinal direction.

1. An active heave compensation system comprising: a motor generator tointeract with a load so as to drive the load in a first part of a heavemotion cycle and to regenerate at least part of energy with which theload has been driven in a second part of the heave motion cycle, anelectrical storage element for storing the regenerated energy, a powersupply electrically connected to the motor generator and the electricalstorage element for providing electrical power to at least the motorgenerator, and a control unit configured to control the power supply tolet an output voltage of the power supply substantially follow, insynchronism with the heave motion cycle, a charging and dischargingvoltage cycle of the electrical storage element.
 2. The active heavecompensation system according to claim 1, wherein the power supply whencontrolled by the control unit, is arranged to provide a substantiallyconstant output current.
 3. The active heave compensation systemaccording to claim 1, wherein the control unit comprises an inputconnected to the power supply for measuring an output parameter of thepower supply, and an output connected to the power supply for drivingthe power supply, the control unit being arranged to compare the outputparameter of the power supply with a control unit setpoint, and drivethe power supply based on the comparison.
 4. The active heavecompensation system according to claim 3, wherein the output parameterof the power supply is an output current of the power supply, andwherein the output of the control unit is connected to a power supplyvoltage setpoint input of the power supply.
 5. The active heavecompensation system according to claim 3, wherein the output parameterof the power supply is a generator frequency signal and wherein theoutput of the control unit is connected to a power supply voltagesetpoint input of the power supply.
 6. The active heave compensationsystem according to claim 3, wherein the control unit is arranged todetermine the control unit set point from: time averaging an actualpower supply output voltage of the power supply, comparing the timeaveraged actual power supply output voltage with a power supply outputvoltage setpoint and deriving the control unit set point from a resultof the comparison.
 7. The active heave compensation system according toclaim 1, wherein the power supply is a current source.
 8. The activeheave compensation system according to claim 7, wherein the control unitcomprises an input connected to the power supply for measuring a powersupply output voltage, and an output connected to the power supply fordriving an output current of the power supply, the control unit beingarranged to determine a time average of the power supply output voltage,compare the time averaged power supply output voltage with a controlunit setpoint, and drive the output current of the power supply based onthe comparison,
 9. The active heave compensation system according toclaim 1, wherein the control unit comprises a heave sensor to measure aheave motion, the control unit being arranged to drive the power supplyfrom the measured heave motion.
 10. The active heave compensation systemaccording to claim 1, wherein the electrical storage element comprises asuper capacitor.
 11. The active heave compensation system according toclaim 10, wherein the electrical storage element comprises a pluralityof super capacitors and a voltage balancing to balance an operatingvoltage of the super capacitors.
 12. The active heave compensationsystem according to claim 10, wherein the super capacitors are arrangedin at least one super capacitor bank, the or each super capacitor bankcomprising two parallel circuit boards, the super capacitors of therespective super capacitor bank being arranged and extending between thecircuit boards.
 13. The active heave compensation system according toclaim 1, wherein the control unit comprises a sensor for measuring avariable representative of a heave motion to be compensated, the controlunit being arranged to drive the motor generator on the basis of themeasured variable.
 14. The active heave compensation system according toclaim 1, wherein the load comprises a vessel motion damping ballastwhich is movable in a transverse or longitudinal direction of a vessel.15. A vessel comprising an active heave compensation system according toclaim
 1. 16. An active heave compensation method comprising: driving theload by a motor generator in a first part of a heave motion cycleregenerating at least part of energy with which the load has been drivenin a second part of the heave motion cycle, storing the regeneratedenergy in an electrical storage element, providing by a power supplyelectrical power to at least the motor generator, the power supply beingelectrically connected to the motor generator and the electrical storageelement, and controlling by a control unit the power supply to let anoutput voltage of the power supply substantially follow, in synchronismwith the heave motion cycle, a charging and discharging voltage cycle ofthe electrical storage element.
 17. The active heave compensation methodaccording to claim 16, wherein the power supply when controlled by thecontrol unit, provides a substantially constant output current.
 18. Theactive heave compensation method according to claim 17, wherein thecontrolling by a control unit the power supply comprises: measuring anoutput parameter of the power supply, comparing the output parameter ofthe power supply with a control unit setpoint, and driving the powersupply based on the comparison.
 19. The active heave compensation methodaccording to claim 18, wherein the output parameter of the power supplyis an output current of the power supply and wherein driving the powersupply based on the comparison comprises driving an output voltagesetpoint input of the power supply.
 20. The active heave compensationMethod according to claim 18, wherein the output parameter of the powersupply is a generator frequency signal and wherein driving the powersupply based on the comparison comprises driving an output voltagesetpoint input of the power supply.
 21. The active heave compensationmethod according to claim 18, wherein the control unit set point isdetermined from time averaging an actual power supply output voltage ofthe power supply, comparing the time averaged actual power supply outputvoltage with a power supply output voltage setpoint, and deriving thecontrol unit set point from a result of the comparison.
 22. The activeheave compensation method according to claim 16, wherein a heave motionis measured by a heave sensor and wherein the power supply is drivenfrom the measured heave motion.